1. Field of the Invention
Because of an explosive increase of IP (Internet Protocol) traffic, there is a rapidly increasing demand for a low cost transmission system having a large capacity. Such a demand may well be responded to by further increasing the capacity and reducing the cost of a WDM (wavelength division multiplexing) transmission system. In respect to the WDM transmission system, the following issues are being addressed as means to further increase the transmission capacity:    (1) achieving a higher transmission rate per wavelength;    (2) high-density wavelength division multiplexing with a narrower interval of signal wavelength placement;    (3) WDM transmission using a plurality of signal wavelength bands (for example, a 1.55-micrometer band (C band) and a 1.58-micrometer band (L band)); and    (4) realization of wider bands by exploiting a new signal wavelength band (for example, an S-band situated in the range of shorter wavelengths than the C-band or an L+-band situated on the side of longer wavelengths than the L-band).
The item (1) is an approach which aims at achieving a larger capacity by raising a signal bit rate per wavelength, and the items (2)–(4) are approaches which aim at achieving a higher capacity by increasing the number of signal wavelengths (i.e., the number of multiplexed wavelengths). As for the issue of cost reduction, a reduction in the number of 3R relays that include an electric-photo-electric conversion is being considered. Namely, optical amplification relaying may be achieved by exclusively using optical amplifiers that are equipped with rare earth-element doped fibers (e.g., erbium doped optical fibers).
When an attempt is made to achieve such a WDM transmission system, a problem arises in that the transmission distance and data-transmission capacity will be limited by limitation factors such as noise generated by optical-fiber amplifiers, chromatic dispersion of optical fibers, polarization mode dispersion (PMD), and accumulation of waveform distortion caused by a nonlinear effect.
The present invention relates to a method of compensating for chromatic dispersion of an optical-fiber transmission path where the method achieves long-distance large-capacity optical transmission in a WDM transmission system.
2. Description of the Related Art
Optical pulses have a wide spectrum in the frequency region when these pulses are generated by controlling the drive current of a light source for outputting light of a given wavelength or when these pulses are generated by modulating intensity of continuous light output from a light source of a certain wavelength by use of an external modulators such as LN (LiNbO3:lithium niobate) modulator. When such optical pulses propagate through an optical fiber having such chromatic dispersion characteristics that the velocity of light propagation is dependent on wavelength, the waveform of optical pulses will be distorted. An effective measure to mitigate the influence of such chromatic dispersion is to control the chromatic dispersion of the optical fiber such as to adjust the dispersion of optical signal wavelengths to almost zero.
A wavelength used for optical transmission of today is approximately 1.55 micrometer, which is the wavelength band used by an optical-fiber amplifier that employs rare earth doped fibers and can directly amplify an optical signal without converting light. In the single mode optical fiber (SMF) that is widely used today, however, zero dispersion wavelength exists around λ=1.3 micrometer. In consideration of this, a dispersion compensation technology that is generally used combines “an SMF used as a transmission path” with “a dispersion compensation fiber (DCF) that has the chromatic dispersion and dispersion slope characteristics of inverse signs relative to the SMF”. Through this combination, the dispersion compensation technology controls an average dispersion of the SMF+DCF in the longitudinal direction to be zero in the signal wavelength band. The WDM transmission-system configuration using this technology is shown in FIG. 1.
FIG. 1 shows an example of a conventional WDM transmission system. The WDM transmission system shown in this illustration includes an optical transmitting apparatus 100, an optical receiving apparatus 200, the optical relaying apparatus 300, and optical transmission lines 400 that connect between these apparatuses.
Light of various wavelengths is output from optical transmitters (OS) 11 provided in the core or the exterior of optical transmitting apparatus 100, and is input, via optical variable attenuators (Variable ATT) 12 for adjusting the input power of optical signals, to multiplexers (MUX) 13, which multiplex the light of various wavelengths. Light of various wavelengths is multiplexed separately for each signal wavelength band (for example, C-band, L-band, and so on). The WDM light signal of each signal wavelength band is amplified by a corresponding erbium-doped fiber amplifier (EDFA) unit 17, which employs an optical fiber with erbium doped therein as a rare earth material. In FIG. 1 and other figures, reference numbers are given to the component parts of only one signal wavelength band for the sake of convenience.
An example of a configuration of a conventional optical amplifier is shown in FIG. 2. The conventional optical-amplifier unit 17 has a configuration in which a dispersion compensation fiber (DCF) 16 having the dispersion and dispersion slope characteristics of reverse signs relative to the transmission-line fibers 400 is provided between the EDFA 14 and the EDFA 15 forming a two-stage configuration. Moreover, FIG. 2 shows an example of a configuration of the EDFA 14. The EDFA 15 has the same configuration as that shown in FIG. 2. The EDFA 14 includes two EDFs, two pump light sources which output pump light, a WDM coupler, an isolator, an attenuator, etc. In this configuration, since the maximum input power to the DCF 16 has to be limited in order to suppress the influence of waveform distortion that is caused by a nonlinear effect generated in DCF 16, the output power of the EDFA 14 situated at the first stage cannot exceed an output power of a certain fixed level. When loss through the DCF 16 becomes large, therefore, an OSNR degradation at the EDFAs 14 and 15 becomes conspicuous, thereby causing a degradation in the optical transmission characteristics of the entire system in addition to various other degradation factors.
Light output from the EDFA 15 of each signal wavelength band is multiplexed by the optical coupler (BAND MUX) 18 that multiplexes light of various signal wavelength bands, followed by propagating into the transmission-line fiber 400. An optical coupler 31 is connected to the output end of the transmission-line fiber 400 accommodated in the optical relaying apparatus 300. Through the optical coupler 31, a pump light source (pump LD) 32 supplies pump light for the purpose of distributed Raman amplification (DRA) that utilizes the transmission-line fiber 400 as an amplification medium by making use of stimulus Raman scattering (SRS). The light that propagates through the transmission-line fiber 400 is thus amplified by DRA, and is input to the optical relaying apparatus 300 situated halfway through the extension of the transmission-line fiber 400.
The light input to the optical relaying apparatus 300 is supplied to optical amplifiers 37 each having a two-stage configuration for a corresponding signal wavelength band, after passing though a band demultiplexer (BAND DEMUX) 33, which demultiplexes the WDM light signal into each wavelength band. Each amplifier 37 is equipped with a DCF 36 between two stages of EDFAs 34 and 35 where the DCF 36 has chromatic dispersion characteristics of reverse signs relative to the transmission-line fiber 400. Light output from the EDFA 35 of each signal wavelength band is multiplexed by a multiplexer 38, and is then input to the transmission-line fiber 400 again.
The light having propagated through the transmission-line fiber 400 while being amplified by DRA is input to the optical receiving apparatus 200, and passes through an optical coupler 21, followed by being demultiplexed into each wavelength band by a demultiplexer 23. Then, the light passes through an amplifier unit 27 having a two-stage configuration comprised of EDFAs 24 and 25, and is input to a demultiplexer (DEMUX) 28, which demultiplexes the light into each wavelength. Light demultiplexed into each wavelength by the demultiplexer 28 is input to and received by an optical receiver 29 of a corresponding wavelength. Through the coupler, a pump light source (pump LD) 22 supplies pump light for the purpose of distributed Raman amplification that utilizes the transmission-line fiber 400 as an amplification medium by making use of stimulus Raman scattering. Accordingly, light that propagates through the transmission-line fiber 400 is amplified by DRA, and is input to the optical receiving apparatus 200 situated at the end of the transmission-line fiber 400.
FIG. 3 shows a schematic diagram of a method of compensating for dispersion. Here, analysis is directed to the accumulated dispersion of a configuration enclosed in a box illustrated in FIG. 3. A graph also shown in FIG. 3 illustrates the accumulated dispersion characteristics of an SMF and a DCF (for C-band or L-band) and those of SMF+DCF. As shown in FIG. 3, the chromatic dispersion characteristics of widely used optical fibers have a characteristic curve showing gradual changes in relation to wavelength. That is, the slope (dispersion slope) of dispersion characteristics is different for each signal wavelength band (C-band, L-band, etc.). Moreover, it is extremely difficult to create DCF that has chromatic dispersion and dispersion slopes of reverse signs relative to transmission-line fibers across all wavelength bands. Accordingly, as shown in FIG. 3, a method generally employed today compensates for chromatic dispersion of transmission paths separately for each signal wavelength band or on a wavelength-specific basis.
As described above, it is possible to bring to zero a total accumulated dispersion of SMF+DCF (as shown by “total” in the FIG. 3) by using a DCF (“DCF for C-band” or “DCF for L-band” in FIG. 3) that has chromatic dispersion characteristics of reverse signs relative to the chromatic dispersion characteristics of an SMF (shown as “SMF” in FIG. 3). Since it is extremely difficult to manufacture DCF that has the chromatic dispersion characteristics of reverse signs relative to transmission-line fibers, however, some residual dispersion may remain in reality.
In conventional optical-wavelength-division-multiplexing systems, transmission-line dispersion is compensated for as described above, thereby suppressing signal waveform degradation caused by chromatic dispersion. In such systems, problems will be encountered as described below when the transmission capacity is increased by raising a bit rate per wavelength or when the transmission capacity is boosted by increasing the number of wavelengths through narrowing of signal wavelength intervals for the merits of high-density wavelength division multiplexing or through exploitation of a new signal wavelength band.
The faster the transmission rate, the broader the spectrum of an optical signal becomes. As a result, an undesirable effect of dispersion and dispersion slopes becomes greater than conventional systems, thereby causing a larger distortion in optical pulses. For this reason, a scheme for dispersion compensation having higher precision than the conventional schemes becomes necessary.
Further, when signal wavelength bands are expanded, a DCF suitable for each transmission path needs to be developed because the dispersion characteristics and the dispersion slope characteristics of optical fibers differ for each signal wavelength band as described above. As an alternative, a plurality of DCFs needs to be combined to provide the dispersion characteristics suitable for the transmission path. Because of such needs, a measure for compensating for dispersion characteristics undesirably becomes complex. When a new signal wavelength band is utilized to increase the number of wavelengths, there is a possibility of accumulated dispersion in this wavelength band becoming large, resulting in a need for an increased number of DCFs. In this case, the conventional configuration as shown in FIG. 1 suffers increasing DCF loss in the DCF 15 arranged between the two stages of the EDFAs 13 and 14 having a two-stage configuration, thereby causing a degradation in the optical signal-to-noise ratio (OSNR). Since there will be stricter requirements for optical receivers as signal speed is increased, an increase in the DCF loss poses a problem when the transmission system having a higher OSNR performance is required. Even in a favorable scenario in which loss at the DCF 16 does not increase, the output power of the EDFA 14 at the first stage needs to be limited in order to suppress a nonlinear effect in the DCF 16, so that an improvement of an OSNR characteristic cannot be expected.
Accordingly, the present invention is aimed at providing an optical-wavelength-division-multiplexing system and a related method that can properly compensate for dispersion while providing high OSNR performance even when a signal-transmission rate is increased or when a signal-transmission bandwidth is expanded, and is also aimed at providing an optical-communication apparatus suitable for implementing such a system.